Whatever you buy, make sure you set it up properly --- the amp puts about 250Wrms into those Kappa's, rated for 75Wrms. That's more power than the speaker was designed to handle. To get the most out of that setup, perhaps consider the following:
+ Match the input sensitivity/gain of the amp and headunit so it doesn't ever sound distorted. Distortion kills speakers and proper matching really helps improve sound by making the most use of the available dynamic range of the system.
+ Set your HPF crossover on the amplifier to match the F0 (resonance frequency) of the speaker (75Hz in case of the Kappa's) to reduce distortion and over-excursion.
+ Make sure the speaker has enough enclosure volume. Speakers are very inefficient so most of the amp's power turns into heat (magnets function as heatsinks too) that needs to dissipate. Excessive heat caused by over-powering, distortion or too small enclosure volume (so heat can't dissipate) reduces the speaker's life.
Hope this helps. Let me know what you end up squeezing in there but make sure it's baffled good, otherwise you're just wasting bass.